1. Field of the Invention
This invention relates generally to magnetic head servo control systems and, more particularly, to disk drive position control systems that determine the location of a head relative to disk tracks.
2. Description of the Related Art
In a conventional computer data storage system having a rotating storage medium, such as a magnetic or magneto-optical disk system, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk, for example, can comprise a disk substrate having a surface on which a magnetic material is deposited. The digital data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from and recorded onto the disk surface in close proximity to the disk. That is, a transducer or read/write head is provided to produce and detect variations in magnetic orientation of the magnetic material as the disk rotates relative to the head.
Conventionally, the read/write head is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head from data track to data track and, once over a selected track, to maintain the head in a path over the centerline of the selected track. Maintaining the head centered over a track facilitates accurate reading and recording of data in the track. Positioning read/write heads is one of the most critical aspects of recording and retrieving data in disk storage systems. With the very high track density of current disk drives, even the smallest head positioning error can potentially cause a loss of data that a disk drive customer wants to record or read. Accordingly, a great deal of effort is devoted to servo systems.
A servo control system generally maintains a read/write head in a position centered over a track by reading servo information recorded onto the disk surface. The servo information comprises a position-encoded servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in disk servo tracks. The flux transitions are recorded as periodic servo pattern bursts typically formed as parallel radial stripes in the servo tracks. When the read/write head passes over the servo pattern flux transitions, the head generates an analog signal whose repeating cyclic variations can be demodulated and decoded to indicate the position of the head over the disk. The demodulated servo signal is referred to as a position error sensing (PES) signal.
In a sector servo method for providing servo track information to a disk servo control system, each disk surface of the disk drive includes servo track information interspersed between binary data recorded in concentric or spiral tracks. The tracks on a sector servo disk surface are divided into radial sectors having a short servo track information area followed by a customer data area. The servo track information area, or sector, typically includes a sector marker, track identification data, and a servo burst pattern. The sector marker indicates the beginning of a servo sector to the data detection electronics, which means that servo information immediately follows in the track. The servo read head can be the same head used for reading data or can be a separate, dedicated servo head. The PES signal is used to generate a corrective input signal that is applied to the read/write head positioning servo.
FIG. 1 is a representation of a conventional quad-burst PES pattern in which magnetic transitions are recorded in bursts labeled as A, B, C, and D. The data tracks are indicated by the track numbers along the left side of the drawing figure. The tracks extend across the page, from right to left. The portion of the disk 22 shown in FIG. 1 extends approximately from track N-1.0 toward the inner diameter of the disk to track N+2.5 toward the outer diameter. Those skilled in the art will appreciate that position information is decoded by demodulating the signal generated by the head passing over the PES burst patterns to form a signal P based on: EQU P=A-b
and to form a quadrature signal Q based on: EQU Q=C-D.
The signals P and Q are quadrature signals because they are cyclic as the head moves across tracks and are out of phase by 90 degrees (one-quarter phase). The magnetic transitions that comprise the PES pattern are represented in FIG. 1 by vertical bars. The letter within each group of bars represents the PES burst recorded therein. One burst is distinguished from another by relative position in a track and relative position to the other bursts. Thus, the signal P should be zero when the head is tracking exactly along the centerline of track N, because the head will receive equal amounts of magnetic field from the A and B servo pattern bursts. A similar situation exists for tracks N+1, N+2, and so forth. For track N+0.5, the signal Q should be zero when the head is tracking exactly along the N+0.5 track centerline because the head will receive equal amounts of field from the C and D servo pattern bursts. The signal Q should be zero also for tracks N+1.5, N+2.5, and so forth.
There is a demand for ever-increasing amounts of storage capacity for customer data. One constraint on the amount of disk surface area for storing customer data is the amount of space required by the PES servo pattern itself. It should be appreciated that every bit of disk surface space freed from servo pattern usage can be shifted to customer data.
From the discussion above, it should be apparent that there is a need for a disk drive system with a PES servo pattern that increases the amount of disk surface area available for storage of customer data. The present invention fulfills this need.